The present invention relates to a TiO.sub.2 pigment particle having a surface coating of inorganic particles wherein improved light scattering efficiency of TiO.sub.2 is achieved at a moderate to a high pigment volume concentration (PVC) when incorporated in coatings applications, and methods of accomplishing the same.
Inorganic pigments, and especially TiO.sub.2, are often incorporated in various polymers as whiteners, tinting agents or opacifiers. TiO.sub.2 is particularly useful for these purposes because it scatters light very efficiently. However a problem encountered is that as the PVC increases in the polymer, the TiO.sub.2 particles come close to or in contact with other TiO.sub.2 particles which reduces the scattering efficiency of the TiO.sub.2, i.e., an optical crowding effect. Typically, scattering efficiency or scattering coefficient is measured by hiding power and tinting strength. While prior art techniques attempt to minimize the foregoing problem, they are deficient in one or more areas. For example, extenders have been added to paint formulations to space the TiO.sub.2 to preserve its scattering efficiency, however, these extenders are difficult to distribute within the paint matrix to maximize the TiO.sub.2 hiding power. Extenders are typically larger particles and/or in the form of aggregates and thus tend to increase the effective TiO.sub.2 volume concentration and decrease scattering efficiency as they are added to replace paint resin content as described in F. Stieg, "The Effect of Extenders on the Hiding Power of Titanium Pigments", Official Digest, 1959, pp. 52-64.
Further, hydrous silica with high surface area can be precipitated onto the surface of the TiO.sub.2 exhibiting some improvement in scattering efficiency at increasing PVC. However, the TiO.sub.2 pigments with hydrous silica coatings with high surface area have a high oil absorption which degrades the paint film integrity, decreases a critical pigment volume concentration (CPVC) as defined hereinbelow and restricts a paint maker of flexibility in formulating paints. In addition, silicas such as fumed silica are composed of many primary particles which are highly aggregated and cannot be distributed uniformly on the TiO.sub.2 surface.
Similarly, use of a polymer coating is expensive and difficult to accomplish and has resulted in severe agglomeration of the TiO.sub.2 particles.
There is a need for a product that has improved scattering efficiency at moderate to high PVC. Concomitantly, there is a need for a TiO.sub.2 having a low oil absorption so that the paint film integrity can be maintained. The present invention meets these needs.